The present invention relates to a fluid machine having a bladed rotor.
When a fluid machine is used in conjunction with a fluid which has erosion characteristics and is therefore likely to wear the surface of the fluid dynamic blade or other surface of the rotor or impeller, it had been known to provide a liner of erosion-resistant material and to attach it to the blade or other fluid dynamic surface of the rotor in order to lengthen the service life of that rotor.
The nature of such wear-resistant liners can be varied and can range from, on the one hand, the use of thin plain mild steel liners attached at localised areas to extensive cover with liners which are coated with an extremely hard wear-resistant material such as tungsten carbide or chromium carbide. The localised plain mild steel type of liner is relatively inexpensive and can be fitted easily. The more extensive liner coated with wear-resistant material tends to be much heavier than the localised thin plain mild steel liner so it can add considerably to the cost and complexity of the rotor or impeller and, due to the unacceptability of welding such hard coated surface material directly to the structural parts of the rotor or impeller, presents additional problems.
The hard-surfaced liner of the blade of the rotor or impeller is particularly difficult to fit, especially in the case of centrifugal impellers, due to fact that the attachment system must be capable of both carrying the centrifugal load induced by the rotating liner and of providing a sufficiently well distributed support to ensure that stresses and deflections within the liner are kept to acceptable limits, even when the rotor is rotating at high speed. It is therefore necessary to ensure that any attachment system does not compromise the structural integrity of the blade as a whole. Such a liner is available in a range of thicknesses which will therefore allow the minimising of the increase in the weight of the rotor or impeller due to the presence of the liner. With such a system there is a relatively high incidence of failure due to the inherent difficulty in reliably attaching such materials to the bladed surface and at the same time causing them to adapt to the shape of the blade surface.
In order to satisfy both of these requirements it has become accepted practice for such liners to be purpose-made to the shape of the blade, and the resulting increase in cost, weight and complexity has had to be accepted. Additionally the requirement for a purpose made liner gives rise to some problems in procuring an adequate supply of the appropriately shaped liner, due to the specialised nature of the manufacture of the liner and to the limited number of potential supplies of such liner.
One prior art form of attachment of a wear-resistant liner relies on the attachment of countersunk bolts into the mild steel backing plate of the purpose made composite liner so as to have their heads entrapped beneath the hard material defining the wear-resistant surface. Such a system is shown in FIG. 3 to be described later.
Typically such a system would use M16 bolts and would require the mild steel backing plate of the liner to have a thickness of the order of 10 mm in order to accommodate the bolt head. Hence the proprietary types of wear resistant plate stock with a hard surface are not suitable for use in this application.
The purpose made liners are often curved and, if so, this curving is carried out prior to depositing the hard surface on the purpose made liner. The pre-formed backing plate is then held in a purpose made jig with all of the embedded countersunk bolts in position, and then the hard surface is laid over the mild steel backing plate to conceal the countersunk heads.
In some environments it may be necessary to provide a protection cup to shield the nut, and the protruding end of the bolt shank, from erosion, as shown in FIG. 3.
U.S. Pat No. 4,565,495 discloses a centrifugal fan having an armouring system comprising a wear resistant liner attached to each blade, for preventing erosion of the aerofoil blades of the fan.
Welding of the stud to the pre-formed liner, as shown in FIG. 4a, is well proven but because the studs need to be very short in relation to their diameter there is difficulty in assuring adequate alignment of the access of the stud shank, and misalignment of only a small fraction of a degree between the nut and the blade surface could result in bending stresses in the bolt shank which exceed the yield stress of the weld and result in detachment of the bolt from the hard surface. Furthermore, any cracks which might be induced in the weld as a result of this bending stress may not be easily detectable prior to start-up of the rotor and detachment of the entire liner may result.
An alternative system (FIG. 4b) of connecting the liner to the blade involves directly plug welding into a recess defined by a hole which is formed in the blade and then blanked off by the (mild steel) backing plate of the composite liner.
In the case of such a plug weld it is difficult to ensure adequate quality of the attachment because of the limited access for both the welding operation and any subsequent inspection operation. There is thus a high risk of weld defects and lack of adequate fusion of the weld. Although such welds can be commonly used successfully for the attachment of static liners where there is negligible induced service load and where the attachment integrity is not so critical, for a liner attached to a dynamic component such as the impeller or rotor blade the need arises for much more critical attachment and the ability to resist high induced service loads.
In order to avoid these disadvantages of the prior art systems it has been necessary to develop alternative methods of attachment of a hard-surfaced liner to a blade of a rotor or impeller so as to ensure that the strength of the attachment is adequate to resist induced stresses arising from both the centrifugal force on the rotating liner and the fluid dynamic forces on the liner/blade combination, and to do so without compromising the structural strength of the blade per se.
Accordingly, one aspect of the present invention provides a rotor for a fluid machine comprising a plurality of blades each provided with a wear-resistant liner attached thereto by welding, characterised in that the weld comprises at least two tack welds between a connector sleeve joined to the backing plate of the liner and a thrust sleeve engaging the rear face of the blade body and holding the thrust sleeve firmly against the rear face of the blade body.
A second aspect of the invention provides a method of connecting a wear-resistant liner to a rotor blade of a fluid machine comprising:- forming a plurality of bores in said rotor blade; attaching a plurality of connector sleeves to the liner at locations to come into register with said bores in the blade when the liner is attached to the blade; the liner being pre-formed to fit on the fluid dynamic surface of the blade; bringing the liner and the blade into engagement and placing a thrust collar around the projecting portion of the connecting sleeve at the rear (non-fluid dynamic) face of the blade; presenting a tubular clamping tool to said thrust collar and mounting it with respect to said connector sleeve so that the tool can be actuated to thrust the thrust collar firmly against the rear surface of the blade body; and applying tack welds between the connector sleeve and the thrust collar to hold the thrust collar in place relative to the connector sleeve, and finally removing the clamping tool from the connector sleeve.